Method of eliminating ion spots on the photo-cathode of electronic optical devices



Aprll 15, 1958 w. HEIMANN 2,831,143

METHOD OF ELIMINATING- ION SPOTS ON THE PHOTO-CATHODE OF ELECTRONICOPTICAL DEVICES Filed July 7, 1954 FIG. 2

Inmemon WALTER HEIMANN (Ittorneg METHUD 01F ELIMINATING ION SPOTS ON THEPHOTO-CATHGDE F ELECTRUNIC GPTEQAL DEVICES Walter Heimann,Wiesbaden-Dotzheim, Germany Application July 7, 1954, Serial No. 441,749

Claims. (Cl. 315-) This invention relates to an improvement inelectronic optical devices, specifically in relation to televisioncamera tubes.

Referring to the drawing, Fig. l is a partial sectional view of anelectronic optical device of the prior art including a photo-cathode,and Fig. 2 is a similar view of an electronic optical device embodyingthe present invention.

The well-known electronic optical device of a television image-dissectortube consists of a flat photo-cathode and a cylinder arranged ahead ofthis cathode which serves as anode. Fig. 1 of the drawing illustratesschematically such a device. At the bottom of a glass cylinder 1 a discshaped photo-cathode 2 is located. A cylindrical anode 3 is placed aheadof the photo-cathode within this glass cylinder. If voltage is applied,the potential gradients generate an accelerating field 4 which acts uponthe electrons released by the cathode and accelerates them in thedirection of the positive anode cylinder. An electrically energized coilwhich surrounds the cylindrical part of the anode tends to arrange theseelectrons in a circular bundle. The coil is not indicated on thedrawing.

Exeprience indicates that in most cases a spot develops on thephoto-cathode after a certain period of usage which gradually leads tothe destruction of the photo-cathode. Investigations have indicated thatthis spot is caused by the continuous bombardment of ions upon thephotocathode. Even if the tube has been fabricated with an almostperfect vacuum, infinitesimal quantities of gas will remain within itwhich will become ionized as soon as the normal working potential of1000 volts is applied. These ions are positively charged and thereforeare directed towards the negative cathode, i. e. in the oppositedirection of the electrons. Fig. 1 indicates the theoretical path 5 ofsome of the ions. As a result of the field potential 4 these ion pathsare concentrated and havea tendency to strike a comparatively. smallarea of the photo-cathode. This concentrated ion bambardment upon asmall area. creates the above mentioned ion spot.

The object of the invention is to prevent the appearance of these spots.The basic idea of the invention consists in the control of the field ofacceleration of electrons ahead of the photo-cathode by means of severaldifferent anode potentials which prevent concentration of positive ionsand achieve, as far as possible, an even and wide distribution of ionswhich strike the entire surface of the photo-cathode. In this manner itis possible to reduce the specific wear of the photo-cathode caused byclashing of ions against it, to the extent that the undesirable effectof the ion spot on the image disappears with corresponding increase inlife of the photo-cathode.

The invention will be more fully set forth in the following descriptionreferring tothe accompanying drawings, and the features of novelty willbe pointed out with particularity in the claims annexed to and forming apart of this specification.

2,831,143 Patented Apr. 15, 1958 The invention solves the abovedescribed condition as schematically illustrated in Fig. 2. As indicatedin this figure, the cylinder representing the anode is divided incylinder sections 3, 6 and 7. The two outer cylinder sections 3 and 7have a positive D. C. potential in relation to the photo-cathode 2 asdescribed above. The center cylinder section 6, however, has a negativeD. C. potential in relation to the two outer cylinders. In addition, thepotential of the center cylinder 6 may be positive or also negative inrelation to the potential of the photo-cathode, and furthermore, it ispossible to change the shape and diameter of the individual anodesections. Through this arrangement of different potentials and/ordiflerent shapes of the anode sections, it is possible to shape thevarious potentials of the accelerating field 4 sothat the paths 5 of theions are no longer concentrated. On the contrary, they are now spreadout in order to obtain, as far as possible, an even distribution of theion charge over the total area of the photocathode.

There is also the possibility to arrange the system of acceleration inonly two stages, i. e. by arranging two anode cylinders ahead of thecathode and by giving the cylinder nearer the cathode a considerablyhigher potential, for instance 1,500 volts as against 1,000 volts of thesecond cylinder.

Depending on the difference in potential between the two cylinders, theions may be dispersed or considerably retarded or may even be reversedin their direction of travel so that they will no longer clash with thecathode.

It should be understood that this invention is not limited to specificdetails of construction and arrangement thereof herein illustrated, andthat changes and modifications may occur to one skilled in the artwithout departing from the spirit of the invention.

What I claim is:

1. Electronic optical apparatus particularly for television camera tubeshaving a plane photo-cathode and a. cylindrical anode arranged ahead ofsaid cathode for the purpose of precluding formation of ion spots on thephotocathode, comprising an anode cylinder divided into at least threeadjacent sections, means applying potentials of different magnitudesbetween adjacent sections for controlling the field of accelerationahead of the photo-cathode and for controlling the spreadinganddispersal of bombarding positive ions over substantially the wholesurface of the photo-cathode thereby to prevent bunching andconcentration of said ions on a relatively small area of the cathode.

2. Electronic optical apparatus as specified in claim 1 wherein theanode consists of at least three cylindrical sections, of which anintermediate section has applied thereto substantially the samepotential as the cathode.

3. Electronic optical apparatus as specified in claim 1 wherein theanode consists of at least three cylindrical sections, and a negativepotential exists between an intermediate section and two adjacentsections.

4. Electronic optical device in accordance with claim 1, characterizedin that a negative potential exists between a center anode cylinder andtwo adjacent cylinders, and that a positive potential exists between thecenter anode cylinder and the cathode.

5. Electronic optical device in accordance with claim 1 characterized inthat a negative potential exists between a center anode cylinder and twoadjacent cylinders as well as between the center anode cylinder and thecathode.

References Cited in the file of this patent UNITED STATES PATENTS2,652,515 McGee Sept. 15, 1953

